1. Field of the Invention
The present invention relates to an image correlation displacement sensor, and more particularly, to an image correlation displacement sensor that captures speckle fields to measure a displacement of an object.
2. Description of Related Art
Image correlation displacement sensors using a correlation between a plurality of speckle fields (speckle images) are known (see U.S. Pat. Nos. 6,642,506; 7,295,324; and 7,515,280). In general, a coherent light source such as a laser light source is used for each image correlation displacement sensor. A speckle field is produced by illuminating an optically rough surface with the coherent light source. Specifically, a target surface is illuminated with coherent light, and light scattered from the target surface is detected by a detector such as a CCD camera or a CMOS image sensor. Thus, the speckle field can be captured in image or images.
First, a speckle field before displacement is captured, and the captured speckle field is stored into a memory or the like as a first speckle image. Next, a speckle field after displacement is captured, and the captured speckle field is stored into a memory or the like as a second speckle image. Then, a displacement of an object to be measured having a target surface is measured based on results of comparison between the first speckle image and the second speckle image.
In the image correlation displacement sensor that measures a displacement by capturing speckle fields, however, the direction of the displacement to be measured is limited. In other words, speckle fields are generally imaged along a direction that is normal to a receiving surface of a detector as well as normal to the target surface, which makes it difficult to measure a displacement along a direction perpendicular to the target surface.
An economical image correlation displacement sensor that accurately measures displacement along a direction including a component perpendicular to a target surface would be desirable.